Hydrazine method of synthesis of γ-Fe₂O₃ useful in ferrites preparation. Part III – study of hydrogen iron oxide phase in γ-Fe₂O₃

Abstract

Direct current electrical conductivity (σ) measurements as a function of temperature have been carried out on γ-Fe₂O₃ prepared from precursors, iron (II) carboxylatohydrazinates, γ-FeOOH and hydrazinated γ-FeOOH. The conductivity variation obeys an Arrhenius equation, ς_I = \ς_oe^-Δ E / kT and the plots of log σ versus 1/T of the as prepared γ-Fe₂O₃, which are in general linear, during the very first heating up to 350°C and cooling to room temperature (RT) do not overlap. This indicates a hysteresis behavior of conductivity, thereby suggesting involvement of two different conductivity mechanisms. When the heat treated sample was equilibrated in a known partial pressure of moisture at ∼200°C and then conductivity measured from RT, the log plots during heating and cooling did not overlap and a hysteresis behavior similar to the as prepared γ-Fe₂O₃ is observed again in the conductivity. Water is considered to be crucial during the synthesis of γ-Fe₂O₃ through magnetite, Fe₃O₄. Protons, H⁺, are thought to be introduced in the spinel Fe₃O₄ making it defective and the oxidation product of this is γ-Fe₂O₃ which retains few protons in its spinel structure. From the structural similarity of such proton incorporated γ-Fe₂O₃ and lithium ferrite, LiFe₅O₈, (Fe³⁺)₈ [Fe³⁺ ₁₂ Li¹⁺ ₄]O₃₂, a formula HFe₅O₈, (Fe³⁺)₈ [Fe³+₁₂H¹+₄]O₃₂ is suggested. A hydrogen iron oxide of formula H_1-xFe_5+x3O₈, where x ∼ 0.1 is probably formed as a maximum limit. Protons are removed during the very first heating of the as prepared sample in the present studies and hence the conductivity of proton free γ-Fe₂O₃ is different and therefore a hysteresis behavior is observed. Moisture equilibration reintroduces the protons. The lithiated samples in the present studies were found to substitute for protons in γ-Fe₂O₃ and no hysteresis behavior is observed in such samples even after moisture equilibration.